In the absence of any external force, liquid crystals will align along the surfaces of samples with their optical axis along one direction, defined as the “easy axis” η0. Conceptually, “anchoring strength” can be thought of as “the amount of force required to cause molecules in the LC at the interface to not lie along the easy axis of the sample.” When the LC is contained between two surfaces which have their easy axes in two different directions, the LC is distorted, such as twisted, splayed or bent. In this distorted configuration, the LC is strained and has an elastic torque force (associated with the LC trying to minimize this strain) and partially “untwists”, if the principle distortion is twist. As the distance between the two surfaces, d, gets smaller this elastic torque gets larger and larger, and the tendency of the liquid crystal to untwist, if the principle distortion is twist, will increase with decrease in d.
With surfaces that have infinitely strong anchoring energy, no amount of torque can cause the liquid crystal at the surface to slip away from the axes of the surfaces. In the case of the finite anchoring, as the elastic torque increases, the orientation of liquid crystal at the surface is dictated by the opposing forces of anchoring strength and elastic torque. The result is a compromised orientation that is φ away from the easy axis. One can estimate the anchoring strength of a surface based on the angle φ as a function of the distance between the two surfaces, d.
A variety of approaches have been explored for the control of the orientation of η0, motivated by advancement in display technologies. The approaches include the oblique deposition of metals, metal oxides and organic materials; the use of mechanically sheared surfaces of organic and inorganic materials, the use of surfaces that are fabricated using photo- and nano-lithography. Many other methods are known by those skilled in the art to prepare surfaces that define an easy axis of a liquid crystal place Equally important for these devices is the ability to manipulate the strength of the interaction between substrate and LC, defined as anchoring energy Waz. The manipulation of azimuthal (in-plane) anchoring energy by tuning the rubbing parameters employed for the treatment of polymeric films is well-documented. However, the method of rubbing simultaneously introduces two elements of substrate structure that can influence LC behavior: 1) microgrooves, or anisotropic topography and 2) anisotropy in the orientation of polymer chains in the near-surface region. Several groups have isolated the contributions from topography, using surface gratings or hard skins that have feature sizes which can be systematically controlled. Conversely, the relationship between surface order and azimuthal anchoring strength has been studied independently of topography via the exposure of photo-alignment layers to linearly polarized light.
Related to the field of manipulation and measurement of azimuthal (in-plane) anchoring energy, the present invention utilizes liquid crystals in constrained geometries and, in certain aspects, offers increased sensitivity to the presence of bound analytes, and forms the basis of quantification of low levels of bound analytes.